Is it coincidence? I wrote a one-pager last month with exactly the same premise!
We could have made a jam out of that ;)
A downloadable game
Marginalia is an asynchronous RPG game made for 2 or more players to be played through the pages of a book. Players will create and adopt worlds and characters in which to tell stories in, and will take turns writing within the margins of an per-existing book.
All that is required for play is a book and a person who has committed to reading it with you!